1. Field of the Invention
The disclosure herein relates generally to methods for determining the composition of a material sample by analyzing electromagnetic energy that has passed through or has been emitted from the material sample.
2. Description of the Related Art
A large number of people suffer from diabetes and other conditions in which the natural regulation of blood glucose levels is impaired. For these people, monitoring blood glucose level is an important part of health maintenance, and a variety of techniques and instruments have been developed to periodically measure glucose levels in blood samples for this purpose.
Most of these methods involve a spectroscopic measurement, where the absorption of electromagnetic energy of a blood sample is measured and correlated to glucose concentration. In some cases, the electromagnetic energy is at optical wavelengths. In these systems, a chemical reagent is typically added to the blood sample which chemically reacts with the glucose and produces an absorption in the optical band dependent on the amount of glucose present and which participates in the reaction. In addition to the expense of manufacturing such reagent based systems, these assays may be interfered with by other blood constituents that reduce their accuracy and reproducibility.
Although reagent based optical assays have been successfully produced and commercialized, blood absorption characteristics in the infrared (IR) region of the electromagnetic spectrum have been recently explored to measure blood glucose concentrations. This has advantages over optical wavelength measurements since glucose exhibits significant absorption in several IR wavelength regions without the need to perform a reaction with another chemical species that must be added to the blood sample.
However, other chemical species including water, alanine, albumin, hemoglobin, urea, lactate and others also absorb strongly at several IR band frequencies. Some of these constituents are present in the blood at concentrations of 50 or 100 times or more than the glucose concentration. Because the sample absorption at any given wavelength is a sum of the absorptions of each component at that wavelength, IR absorption measurements are complicated by the presence of these other components. Consequently, methods that allow effective compensation and adjustments to measured IR absorption for the presence of other blood components would be beneficial to provide a low cost and accurate system for diabetics and others in need of periodic glucose monitoring.
3. Summary of the Invention
In accordance with certain embodiments described herein, a method determines an analyte concentration in a sample comprising the analyte and a substance. The method comprises providing an absorption spectrum of the sample. The absorption spectrum has an absorption baseline. The method further comprises shifting the absorption spectrum so that the absorption baseline approximately equals a selected absorption value in a selected absorption wavelength range. The method further comprises subtracting a substance contribution from the absorption spectrum. Thus, the method provides a corrected absorption spectrum substantially free of a contribution from the substance.
In accordance with other embodiments described herein, a method provides pathlength-insensitive measurements of blood constituents in a sample using infrared (IR) spectroscopy. The method comprises providing an absorption spectrum of the sample. The absorption spectrum has an absorption baseline. The method further comprises shifting the absorption spectrum so that the absorption baseline approximately equals a selected absorption value in an absorption wavelength range comprising an isosbestic wavelength at which water and a whole blood protein have approximately equal absorptions.
In accordance with still other embodiments described herein, a method measures a pathlength of a sample comprising a first component and a second component. The method comprises providing an absorption spectrum of the sample. The method further comprises determining an absorption value of the absorption spectrum at an isosbestic wavelength at which the first component and the second component have approximately equal absorptions. The method further comprises calculating the pathlength from the absorption value.
In accordance with still other embodiments described herein, a method estimates a glucose concentration of a blood sample. The method comprises measuring sample absorption at a plurality of wavelengths between about 4 microns and 11 microns. The method further comprises subtracting a contribution to said sample absorption due to the presence of water. The method further comprises subtracting a contribution to said sample absorption due to the presence of hemoglobin.
For purposes of summarizing the invention, certain aspects, advantages and novel features of the invention have been described herein above. It is to be understood, however, that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.